Revisiting the Oxidation State of [Cu(HIO₆)₂]^5-: A Copper(III) Complex between Classical Werner-Type and Inverted...

Area of Science:

• Inorganic Chemistry
• Materials Science
• Spectroscopy

Background:

• The electronic structure of copper(III) complexes is debated, with models including classical Werner-type (3d^8) and inverted ligand field (3d^10 Cu(I)) frameworks.
• The copper periodate complex [Cu(HIO_6)_2]^(5-) has been historically considered a 3d^8 Cu(III) species, serving as a reference for Cu-containing metalloenzymes.
• However, its detailed electronic structure and oxidation state have remained unexplored.

Purpose of the Study:

• To investigate the oxidation state and electronic structure of the copper periodate complex [Cu(HIO_6)_2]^(5-).
• To determine if the complex fits classical Werner-type (3d^8) or inverted ligand field (3d^10 Cu(I)) descriptions.
• To challenge the adequacy of binary classifications for Cu(III) complex electronic states.

Main Methods:

• X-ray photoelectron spectroscopy (XPS) to probe electronic binding energies.
• X-ray absorption spectroscopy (XAS) to analyze electronic transitions and coordination environment.
• Density functional theory (DFT) calculations to model electronic structure and bonding.

Main Results:

• The electronic structure of the copper center in [Cu(HIO_6)_2]^(5-) was found to be intermediate.
• Results indicate the complex exists at the boundary between classical Werner-type and inverted ligand field regimes.
• The study provides experimental and theoretical evidence for this complex electronic state.

Conclusions:

• Categorizing Cu(III) complexes strictly as 3d^8 or 3d^10 is often insufficient.
• Electronic states at the boundary between classical and inverted ligand field models are recognized in Cu(III) complexes.
• This finding necessitates a more nuanced understanding of copper oxidation states in inorganic complexes.